Beamed Sails: The Problem with Lasers

byPaul GilsteronAugust 25, 2014

We saw on Friday through Jim Benford’s work that pushing a large sail with a neutral particle beam is a promising way to get around the Solar System, although it presents difficulties for interstellar work. Benford was analyzing an earlier paper by Alan Mole, which had in turn drawn on issues Dana Andrews raised about beamed sails. Benford saw that the trick is to keep a neutral particle beam from diverging so that the spot size of the beam quickly becomes much larger than the diameter of the sail. By his calculations, only a fraction of the particle beam Mole envisaged would actually strike the sail, and even laser cooling methods were ineffective at preventing this.

It seems a good time to look back at Geoffrey Landis’ paper on particle beam propulsion. I’m hoping to discuss some of these ideas with him at the upcoming Tennessee Valley Interstellar Workshop sessions in Oak Ridge, given that Jim Benford will also be there. The paper is “Interstellar Flight by Particle Beam” (citation below), published in 2004 in Acta Astronautica, a key reference in an area that has not been widely studied. In fact, the work of Mole, Andrews and Benford, along with Landis and Gerald Nordley, is actively refining particle beam propulsion concepts, and what I’m hoping to do here is to get this work into a broader context.

Particle beams are appealing because they solve many of the evident limitations of laser beaming methods. To understand these problems, let’s look at their background. The man most associated with the development of the laser sail concept is Robert Forward. Working at the Hughes Aircraft Company and using a Hughes fellowship to assist his quest for degrees in engineering (at UCLA) and then physics (University of Maryland), Forward became aware of Theodore Maiman’s work on lasers at Hughes Research Laboratories. The prospect filled him with enthusiasm, as he wrote in an unfinished autobiographical essay near the end of his life:

“I knew a lot about solar sails, and how, if you shine sunlight on them, the sunlight will push on the sail and make it go faster. Normal sunlight spreads out with distance, so after the solar sail has reached Jupiter, the sunlight is too weak to push well anymore. But if you can turn the sunlight into laser light, the laser beam will not spread. You can send on the laser light, and ride the laser beam all the way to the stars!”

The idea of a laser sail was a natural. Forward wrote it up as an internal memo within Hughes in 1961 and published it in a 1962 article in Missiles and Rockets that was later reprinted in Galaxy Science Fiction. George Marx picked up on Forward’s concepts and studied laser-driven sails in a 1966 paper in Nature. Remember that Forward’s love of physical possibility was accompanied by an almost whimsical attitude toward the kind of engineering that would be needed to make his projects possible. But the constraints are there, and they’re formidable.

Landis, in fact, finds three liabilities for beamed laser propulsion:

The energy efficiency of a laser-beamed lightsail infrastructure is extremely low. Landis notes that the force produced by reflecting a light beam is no more than 6:7 N/GW, and that means that you need epically large sources of power, ranging in some of Forward’s designs all the way up to 7.2 TW. We would have to imagine power stations built and operated in an inner system orbit that would produce the energy needed to drive these mammoth lasers.

Because light diffracts over interstellar distances, even a laser has to be focused through a large lens to keep the beam on the sail without wasteful loss. In Forward’s smaller missions, this involved lenses hundreds of kilometers in diameter, and as much as a thousand kilometers in diameter for the proposed manned mission to Epsilon Eridani with return capability. This seems highly impractical in the near term, though as I’ve noted before, it may be that a sufficiently developed nanotechnology mining local materials could construct large apertures like this. The time frame for this kind of capability is obviously unclear.

Finally, Landis saw that a laser-pushed sail would demand ultra-thin films that would need to be manufactured in space. The sail has to be as light as possible given its large size because we have to keep the mass low to achieve the highest possible mission velocities. Moreover, that low mass requires that we do away with any polymer substrate so that the sail is made only of an extremely thin metal or dielectric reflecting layer, something that cannot be folded for deployment, but must be manufactured in space. We’re a long way from these technologies.

This is why the particle beam interests Landis, who also looked at the concept in a 1989 paper, and why Dana Andrews was drawn to do a cost analysis of the idea that fed into Alan Mole’s paper. Gerald Nordley also discussed the use of relativistic particle beams in a 1993 paper in the Journal of the British Interplanetary Society. Here is Landis’ description of the idea as of 2004:

In this propulsion system, a charged particle beam is accelerated, focused, and directed at the target; the charge is then neutralized to avoid beam expansion due to electrostatic repulsion. The particles are then re-ionized at the target and reflected by a magnetic sail, resulting in a net momentum transfer to the sail equal to twice the momentum of the beam. This magnetic sail was originally proposed to be in the form of a large superconducting loop with a diameter of many tens of kilometers, or “magsail” [7].

The reference at the end of the quotation is to a paper by Dana Andrews and Robert Zubrin discussing magnetic sails and their application to interstellar flight, a paper in which we learn that some of the limitations of Robert Bussard’s interstellar ramjet concept — especially drag, which may invalidate the concept because of the effects of the huge ramscoop field — could be turned around and used to our advantage, either for propulsion or for braking while entering a destination solar system. Tomorrow I’ll continue with this look at the Landis paper with Jim Benford’s findings on beam divergence in mind as the critical limiting factor for the technology.

So the economic argument boils down to the useful energy that can be extracted by the vehicle. Laser light is inefficient to produce, but beam divergence ensures a higher fraction reaches the sail. Particle beams are cheaper, but divergence reduces the delivered power. Do the relative tradeoffs differ for beam distance? If we use microwaves instead, where does this technique stand in comparison?

I’m also a bit concerned over the need of the vehicle to actively re-ionize the neutral beams. How much of the received beamed energy must be converted to power for these ionizers, reducing the propulsion? Also how much damage to the vehicle will particle beams cause, after all these were going to be SDI weapons to bring down ICBMs?

The “size of infrastructure” argument against laser beams doesn’t really impress me. Any civilization capable of building starships is probably going to be capable of building some truly impressive space infrastructure in their own solar system.

I find the suggestion of using neutral particle beams that require 300 GWe space power source that is much more efficient then a laser pushed sail less credible the using a Sonny White warp drive for achieving 1 c vs 2c spacecraft speeds. The reason being with 31 years of conventional and advanced space power system development [including a 1GWe Space Based Solar Power System] preceded by 17 years of advanced nuclear power R&D the killer issue is how to deploy a minimum 300 GWt radiator for the space power system not to mention the propellant needed to maintain the hugh space vehicle’s configuration with the impinging solar flux!

Hi Paul,
There were some discussions over the years regarding means to increase the efficiency of laser sails. The basic concept is that the laser beam would be reflected from the sail back to a mirror (presumably attached to a substantial mass) which would reflect the beam back to the sail. If it were possible, the photons could be reflected back and forth thousands of time each time imparting momentum both to the sail and to the mirror. The sail, being much lighter, would undergo the bulk of the acceleration. The beam would end up transferring thousands of times more momentum than a single reflection typical of most laser sail concepts.

The beam would spread with each reflection unless there were means to refocus the beam and some photons would be absorbed in the mirrors. The driving laser would make up these losses. Even an average of 10 reflections would reduce the net power consumption of the laser ten-fold. Is there something that fundamentally limits the potential of this concept; diffraction? Relatively poor reflective efficiency, aiming accuracy, very high beam intensities due to multiple reflections? Just wondering. Thanks.

So what is the maximum speed one might hope to achieve using just the Sun (no lasers)? 1% c? 10%? Assume a crewed spacecraft with the mass of a trust submarine.

I can’t make the calculation because I have’re studied calculus etc yet. :)

But if the Sun can push a crewed spacecraft fast enough to get to the nearest star in 200 years or less I think that would be fine, just put people in hibernation until they get there.

Only thing that stops banned propulsion from being ideal is that you can’t easily change course and if you use a laser it particle bean instead of just the Sun, the politicians might turn off the bean! :-o

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last eleven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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